Carroll said that he never went through a religion deconversion moment; instead he had a long journey that led him to disbelief. He first started having strong doubts about Catholicism when he went into Seminary in Notre Dame. He became intrigued by eastern religions after leaving the Seminary. Inspired by Alan Watts, he started looking at eastern religions' holy books. Carroll became interested in Paramahansa Yogananda and attended his Self-Realization Fellowship group to do yoga and chanting, he identified as agnostic at the time. After leaving the fellowship, he spent many years thinking in depth about his religion. He said "The more I thought about religious ideas, the more false and absurd they seem to me." Carroll became enchanted by the ideas of Kierkegaard, namely the idea that religious beliefs require a leap of faith because they cannot be rationally proven. Instead, Carroll decided to take a leap in the other direction: he said that he "found many reasons for disbelief and absolutely no reasons for belief", and that his former religious beliefs lost their attractiveness.[11]

Carroll's teaching career started with a part-time position teaching philosophy at Lassen Community College. He taught philosophy of religion at American River College for two years before going full-time in Sacramento City College where he taught introductory philosophy classes, logic and critical reasoning, law justice and punishment and critical thinking about the paranormal for three decades (1977–2007) as well as serving several years as chairman.[1][15][16][17]

Carroll authored Becoming a Critical Thinker, a textbook for introductory logic and critical thinking courses. It covers subjects such as language and critical thinking, the mass media and other sources of information, fallacies of reasoning, and inductive and deductive arguments.[18] The book is subtitled A Guide for the New Millennium. Pearson Educational published the first edition in 2000 and the second edition was published in 2005.[1]Becoming a Critical Thinker was born out of Carroll's classwork during his time in Sacramento City College.[11]

The Skeptic's Dictionary is the print version of the website skepdic.com and is available in Dutch, English, Japanese, Korean, and Russian.[19] It provides definitions, arguments, and essays on supernatural, occult, paranormal, and pseudo-scientific subjects.[20] The book features many examples of pseudoscientific beliefs over its eight chapters. In the last chapter, Carroll provided ways to improve critical thinking and skepticism.[21] Similar to the website, it takes a skeptical stance, typically assuming that something is false until proven otherwise.[22][23] The book came about when Ted Weinstein, a literary agent, contacted Carroll about creating the book. The book was eventually published by John Wiley & Son in August 2003 as an inexpensive paperback.[18] The book is intended to be biased towards the skeptical side;[19] it is not targeted towards true believers.[24]

Carroll also wrote a children's version of the Skeptic's dictionary which was released online on July 22, 2011. In 2013, it was published as a children's book under the title Mysteries and Science: Exploring Aliens, Ghosts, Monsters, the End of the World and Other Weird Things. He also wrote Unnatural Acts: Critical Thinking, Skepticism, and Science Exposed! which was initially published by the James Randi Educational Foundation as an e-book in 2011. A paperback version is available from Lulu.[1]The Critical Thinker's Dictionary was published in 2013. It features short articles about cognitive biases and logical fallacies.[25]

A long-time advocate of scientific skepticism and critical thinking, Carroll said that he had been investigating controversial beliefs since he was seven years old when he had doubts about Santa Claus.[3] Carroll described the importance of critical thinking and open-mindedness in the following quotation:

"If you are willing to be open minded, accept that reasonable probabilities rather than absolute certainties are the best information in many things that matter. And hold your most precious beliefs tentatively, then you can overcome some of the hindrances to critical thinking at least some of the time. And also that one's world view can be a major hindrance to being fair minded. The minimum requirement of fair mindedness is a willingness to take seriously viewpoints opposed to your own. In other words, you have to be willing to admit that you are wrong. Or that you might be wrong"

Carroll's background in philosophy meant that he was well versed in philosophical skepticism. His area of specialization was 17th century British philosophy, namely epistemology and philosophy of religion. Carroll's skeptical career started with his teaching career. However, he did not start writing skeptical content until 1992 when both Carroll's best friend and his father-in-law died within the same week. These circumstances profoundly affected Carroll and made him realize his own mortality, providing the catalyst that started his skeptical writing.[27] In an interview with Beyond a Doubt's Blair Anthony Robertson, Carroll described the influence of this incident: "It was like the deaths of these two people had forced me to start looking at everything and not take anything for granted. All these times I had said that someday I am going to do this or go there, I decided to just do it."[28]

In 1994, Carroll and his wife attended free training organized by Davis Community Network in which they learned about the Internet and e-mail as well as HTML code.[29][30] Shortly after, Carroll started the Skeptic's Dictionary website with just ten articles that he had written for his students about logic and pseudoscience.[11][15] The website is considered among the first to use hypertext.[3][28] The potential for including links to external materials was part of the appeal of publishing on the internet for Carroll. As of April 2016[update], the website has more than 5,500 files and 85,000+ hyperlinks.[24] The website had been a one-man project since its inception. Although it never had a staff behind it, several translators volunteered to translate it into more than a dozen languages and several volunteer editors have provided assistance.[18][31] In Carroll's mind, The Skeptic's Dictionary wasn't an encyclopedia.[15] He saw it as a source of skeptical arguments intended for skeptics to counterbalance the arguments on the other side rather than being targeted towards true believers.[20] As of April 2016[update], the site had grown to 784 articles that included definitions, arguments, and essays on subjects supernatural, occult, paranormal, and pseudoscientific.[20][24] As of April 2016[update], it attracted more than four hundred thousand visitors per month.[32] The Skeptics’ Dictionary was inspired by Pierre Bayle's Historical and Critical Dictionary in both its name[15] and its biased stance. Carroll was most intrigued by the Baruch Spinoza entry in Bayle's dictionary; he described its influence in an interview with Point of Inquiry's Karen Stollznow:

"Bayles's dictionary was definitely biased in the sense that he would do a biography or entry on like say atheism, and his entry on atheism would be an essay and the essay would question common beliefs about atheism mainly being that atheists are immoral libertines and so on, and so his essay on atheism held Spinoza as the model atheist"

Carroll created skeptical blogs such as Mass Media Bunk and Mass Media Funk (they were replaced by Skeptimedia in 2007). He also maintained Skeptimedia and Suburban Myths blogs in addition to skepdic.com.

On March 27, 2012, Carroll began a regular segment on the podcast Skepticality entitled Unnatural Virtue in which he commented on topics in critical thinking and skepticism.[33][34] The segment ran until April 29, 2014 for thirty-one episodes.

Carroll spoke at several skeptic conferences. In 2003, he was invited to give a talk at the first Amaz!ng Meeting. In the same year he gave a speech at the Committee for the Scientific Investigation of Claims of the Paranormal conference on frauds and hoaxes. In 2004, he gave a speech to the Irish Skeptics in Dublin on scientific proof for the paranormal.[1] In 2007, he conducted a critical thinking workshop at the 5th Amazing Meeting. On May 29, 2011, Carroll led a discussion concerning the "Five Myths About Skeptics" at the 2nd annual SkeptiCalCon event held in Berkeley, CA.[35]

In an interview with Point of Inquiry's Karen Stollznow, Carroll hinted that he did not earn a substantial amount of money from his skeptical work: "If we talk about the money we make from skepticism we might set a record for the shortest interview ever." However, he insisted that everybody should be a skeptic because, he said, it is a healthy way of approaching life. He said that skeptics' meetups and conferences, as well as the positive feedback he received on his work were his main motivations.[17]

In May 2016, Carroll announced that he would no longer be able to write the Skeptic's Dictionary monthly newsletter on account of his illness.[40]

After Carroll wrote a Skepdic.com entry about Jon Barron criticizing his "Barron Effect",α Barron replied with a piece on his website titled "Rebutting a Skeptic" in which he replied to Carroll's Skepdic.com entries concerning alternative medicine such as "Cellular energy's relation to cancer"; "Mercury, thimerosal, vaccines, and chelation"; "The need for supplements" and "Fecal matter in the colon". In the piece he accused Carroll of deliberately misinterpreting evidence, as well as taking issue with a distorted personal picture that Carroll had posted on skepdic.com.[41]

After Carroll published a piece online labelling Richard Milton's writings on alternative science as "Internet Bunk", Milton responded by accusing Carroll of being a "pseudo-skeptic", and claimed to show that Carroll had fabricated quotations and misrepresented his arguments.[42] Carroll countered these accusations in an addendum to his piece.[43]

Carroll wrote two Skeptic's Dictionary entries that criticize Rupert Sheldrake's ideas. The first criticized Sheldrake's N'kisi Project, which is a set of experiments that claim to have proven the existence of a telepathic link between N'kisi (a grey parrot) and its owner Aimee Morgana. Carroll criticized Sheldrake for omitting 60% of the data he collected when calculating the statistical significance of the parrot's responses.[44][45][46] Carroll also wrote an entry in his Mass Media Bunk blog criticizing Jane Goodall for her involvement in the N'kisi Project.[47] The second entry challenged Sheldrake's morphic resonance idea, in which Sheldrake proposed that each species has a "morphogenetic field" that evolves similarly to how the species' genes might evolve. He then proposed that these fields organize the nervous system's activity and can act as a collective memory for the whole species, and that these fields get passed down into the species by a kind of resonance called morphic resonance.[46][48][49]

Sheldrake replied to Carroll's criticism by writing an article on his website defending his own arguments and accusing Carroll of committing several logical fallacies, misusing his scientific knowledge, misinterpreting and manipulating data, as well as being prejudiced. He also criticized The Skeptic's Dictionary, claiming that it would not survive had it been subject to independent peer reviews.[46]

Carroll did not believe in an omnipotent, omniscient, omnipresent god.[39] However, in his essay "Why I am not an atheist", Carroll described his dislike of the term "atheist" because he felt that the term was being exploited by theists and used as a straw man argument. He felt that the term implied a dogmatic set of beliefs and carried its own share of negative baggage. So, Carroll suggested that atheists might as well adopt the term "Brights" with all its negative connotations.[50]

The only religion that Carroll found attractive after abandoning Catholicism, despite never following it, was Buddhism as taught by the Dalai Lama.[11]

Carroll always maintained the opinion that people have to be more skeptical of religion than they are now.[3] He said in multiple interviews that religion is an area that skeptics don't target enough,[3][27][51] and that pure faith is winning the race against critical thinking.[18]

Carroll tended to have a moderate outlook on religion. He believed that religion has a role to play in people's lives and he didn't condemn religion for terrorism. When asked about the relationship between violence and religion he said that he can't recall anything negative about his religious upbringing, and that maybe Catholicism can provide more good than harm. He didn't believe religion causes wars, he rather believed that it serves as an excuse for people who will go to war regardless of religion's existence.[11] Carroll believed that some people rely on religion as their only source of morality and as a source of comfort.[27] However, he found it distressing that some people are unable to find meaning in their lives without religion.[3] In an interview with Beyond a Doubt he said

"There is nothing dull about a life without fairies, Easter bunnies, devils, ghosts, magic crystals, etc. Life is only boring to boring people."